White heat-curable silicone resin composition and optoelectronic part case

ABSTRACT

A white heat-curable silicone resin composition comprising (A) a heat-curable organopolysiloxane, (B) a white pigment, (C) an inorganic filler, and (D) a condensation catalyst cures into a white uniform product having heat resistance, light resistance and minimal yellowing. The cured composition has a thermal conductivity of 1-10 W/mK. The composition is useful for forming cases on optoelectronic parts, typically LED.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application Nos. 2008-150328 and 2008-150356 filed in Japan onJun. 9, 2008 and Jun. 9, 2008, respectively, the entire contents ofwhich are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to white heat-curable silicone resin compositionsfor forming optoelectronic part cases, featuring whiteness, heatresistance, light resistance, uniformity and minimal yellowing, andimparting a cured product having a high thermal conductivity. It alsorelates to cases for receiving optoelectronic parts, typically LED, thecases being formed of the compositions in the cured state.

BACKGROUND ART

Optical semiconductor parts such as light-emitting diodes (LED) havemany advantages including small size, efficiency, vivid color emission,elimination of bulb failure, excellent drive characteristics, resistanceto vibration, and resistance to repeated turn-on and off. These partsare thus often utilized as various indicators and light sources.Optoelectronic devices using optical semiconductor parts are enclosed incases or packages, which are now typically made of polyphthalamide (PPA)resins.

The current rapid advance of the photo-semiconductor technology hasbrought about photo-semiconductor devices of increased output andshorter wavelength. Photo-semiconductor devices are often encapsulatedor encased using prior art PPA resins as colorless or white material.However, these encapsulants and cases are substantially degraded duringlong-term service and susceptible to visible color shading, separationand a lowering of mechanical strength. It is desired to overcome theseproblems effectively.

More particularly, JP 2,656,336 discloses that an optoelectronic deviceis encapsulated with a B-staged epoxy resin composition, in the curedstate, comprising an epoxy resin, a curing agent, and a cure promoter,the components being uniformly mixed on a molecular level. As to theepoxy resin, it is described that bisphenol A epoxy resins or bisphenolF epoxy resins are mainly used although triglycidyl isocyanate and thelike may also be used. In examples, a minor amount of triglycidylisocyanate is added to the bisphenol A or F epoxy resin. The presentinventors have empirically found that this B-staged epoxy resincomposition for semiconductor encapsulation tends to yellow when held athigh temperatures for a long period of time.

Triazine derivative epoxy resins are used in LED-encapsulating epoxyresin compositions as disclosed in JP-A 2000-196151, JP-A 2003-224305,and JP-A 2005-306952. None of these patents succeed in solving theproblem of yellowing during long-term service at high temperature.

JP-A 2006-140207 discloses a light reflective resin composition having ahigh thermal conductivity and a light reflectance of at least 80% in thewavelength region of 800 to 350 nm. Since this composition is based onan epoxy resin, it suffers from the problem that yellowing can occurduring long-term service at high temperature, particularly when the LEDto be encapsulated in the composition is a UV, white, blue or similarLED of the high intensity type.

JP-A 2006-077234 describes a LED-encapsulating resin compositioncomprising an organopolysiloxane having a weight average molecularweight of at least 5×10³ and a condensation catalyst. Since thisorganopolysiloxane must be transparent and liquid at room temperature,the composition does not lend itself to transfer molding and compressionmolding.

The heat released from LED is one factor that can degrade transparentencapsulants and reflectors used in LED packages and thus causes a lossof intensity or luminance. There is a need for a material having a highreflectance and a high thermal conductivity or heat resistance.

At the present, cases for optoelectronic parts such as LED are generallymanufactured by mounting a LED chip on a lead frame, wire bonding,molding an epoxy or silicone resin thereon for encapsulation, andfurther molding a reflector material thereon to form an enclosure orreflector for preventing any escape of light from LED. The resincomposition for enclosure must be adherent to the lead frame.Conventional enclosing resins, however, are less adherent to leadframes. Specifically, in the mounting process including a solder reflowstep, the assembly is exposed to such high temperatures (215 to 260° C.)that separation may occur at the interface between the lead frame andthe resin. Lack of reliability is a serious problem.

CITATION LIST

-   Patent Document 1: JP 2,656,336-   Patent Document 2: JP-A 2000-196151-   Patent Document 3: JP-A 2003-224305-   Patent Document 4: JP-A 2005-306952-   Patent Document 5: JP-A 2006-140207-   Patent Document 6: JP-A 2006-077234-   Patent Document 7: JP-A 2007-235085-   Patent Document 8: JP-A 2007-297601

SUMMARY OF INVENTION

An object of the invention is to provide a white heat-curable siliconeresin composition for forming optoelectronic part cases, which curesinto a white uniform product having heat resistance, light resistance,minimal yellowing and high thermal conductivity; and a case foroptoelectronic parts, typically LED, made of the cured composition.

The inventors have found that a white heat-curable silicone resincomposition comprising (A) a heat-curable organopolysiloxane, (B) awhite pigment, (C) an inorganic filler excluding the white pigment, and(D) a condensation catalyst as essential components cures into a whiteuniform product having heat resistance, light resistance and minimalyellowing as well as a high thermal conductivity, and is thus useful forforming cases on optoelectronic parts, typically LED. This is trueparticularly when the cured composition has a thermal conductivity of 1to 10 W/mK.

The inventors have also found that a white heat-curable silicone resincomposition further comprising (F) a silane coupling agent, and/or (G)an adhesive aid which is an epoxy resin having the compositional formula(2), defined below, cures into a product which is fully adherent to asubstrate as demonstrated by a bond strength of at least 4 MPa and isthus useful for forming cases on optoelectronic parts.

The present invention is a white heat-curable silicone resin compositionfor forming optoelectronic part cases, comprising as essentialcomponents,

(A) a heat-curable organopolysiloxane,

(B) a white pigment,

(C) an inorganic filler excluding the white pigment, and

(D) a condensation catalyst,

the composition having a thermal conductivity of 1 to 10 W/mK.

In a preferred embodiment, the white pigment (B) is one or more membersselected from the group consisting of titanium dioxide having an averageparticle size of 0.05 to 5.0 μm, and potassium titanate, zirconiumoxide, zinc sulfide, zinc oxide, alumina, and magnesium oxide, eachhaving an average particle size of 0.1 to 3.0 μm, and the inorganicfiller (C) is one or more members selected from the group consisting ofalumina, zinc oxide, silicon nitride, aluminum nitride and boronnitride, each having an average particle size of 4 to 40 μm.

In a preferred embodiment, a powder mixture of the white pigment (B) andthe inorganic filler (C) has a particle size distribution having maximumpeaks in the three ranges of 0.4 to 1.0 μm, 8 to 18 μm, and 30 to 50 μm.

In another preferred embodiment, the white pigment (B) and the inorganicfiller (C) are present in a total amount of 50 to 95% by weight based onthe total weight of the composition.

In a preferred embodiment, the heat-curable organopolysiloxane (A) hasthe average compositional formula (1):R¹ _(x)Si(OR²)_(y)(OH)_(z)O_((4-x-y-z)/2)  (1)wherein R¹ is each independently an organic group of 1 to 20 carbonatoms, R² is each independently an organic group of 1 to 4 carbon atoms,x, y and z are numbers satisfying 0.8≦x≦1.5, 0≦y≦0.3, 0.001≦z≦0.5, and0.801≦x+y+z<2.

Typically, the white pigment (B) is titanium dioxide.

The composition may further comprise (E) a parting agent comprisingcalcium stearate having a melting point of 120 to 140° C. and is presentin an amount of 0.2 to 5.0% by weight based on the total weight of thecomposition.

The composition may further comprises (F) a silane coupling agent and/or(G) an adhesive aid which is a 1,3,5-triazine nucleus derivative epoxyresin having the compositional formula (2):

wherein R⁰¹, R⁰² and R⁰³ each are an organic group of 1 to 10 carbonatoms, at least one of R⁰¹, R⁰² and R⁰³ containing an epoxy group.

In this case, the condensation catalyst (D) is preferably anorganometallic condensation catalyst. The organometallic condensationcatalyst is preferably zinc benzoate.

Also provided is an optoelectronic part case comprising the whiteheat-curable silicone resin composition of the third embodiment in thecured state, in which a transparent resin-encapsulated optoelectronicpart is enclosed.

ADVANTAGEOUS EFFECTS OF INVENTION

Since the white heat-curable silicone resin compositions according tothe invention cure into white uniform products having heat resistance,resistance to light (including emission from optoelectronic parts) andminimal yellowing and exhibiting a high thermal conductivity, they areuseful in forming cases for enclosing optoelectronic parts such as LED.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates an optoelectronic device in a case (inthe form of LED reflector) made of the heat-curable silicone resincomposition of the invention.

DESCRIPTION OF EMBODIMENTS

The singular forms “a,” “an” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used herein, the term “average particle size” is determined as weightaverage value D₅₀ or median diameter upon particle size distributionmeasurement by the laser light diffraction method.

The term “phr” refers to parts by weight per 100 parts by weight ofcomponent (A) unless otherwise stated.

The invention provides a white heat-curable silicone resin compositionfor forming optoelectronic part cases, comprising as essentialcomponents, (A) a heat-curable organopolysiloxane, (B) a white pigment,(C) an inorganic filler excluding the white pigment, and (D) acondensation catalyst, the composition having a thermal conductivity of1 to 10 W/mK.

Now the components used are described in detail.

A. Organopolysiloxane

Component (A) is a heat-curable organopolysiloxane, specifically asilanol-containing organopolysiloxane, and more specifically a siliconepolymer having the average compositional formula (1):R¹ _(x)Si(OR²)_(y)(OH)_(z)O_((4-x-y-z)/2)  (1)wherein R¹ is each independently an organic group of 1 to 20 carbonatoms, R² is each independently an organic group of 1 to 4 carbon atoms,x, y and z are numbers satisfying the range: 0.8≦x≦1.5, 0≦y≦0.3,0.001≦z≦0.5, and 0.801≦x+y+z<2.

The organic groups represented by R¹ include substituted orunsubstituted monovalent hydrocarbon groups of 1 to 20 carbon atoms, forexample, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₆-C₂₀ aryl, and C₇-C₂₀ aralkylgroups. Of the alkyl groups, C₁-C₁₀, alkyl groups are preferred and maybe straight, branched or cyclic. Examples include methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl,hexyl, octyl, cyclopentyl, and cyclohexyl. Of the alkenyl groups, C₂-C₁₀alkenyl groups are preferred, examples of which include vinyl, allyl andpropenyl. Of the aryl groups, C₆-C₁₀ aryl groups are preferred, examplesof which include phenyl, tolyl, xylyl and naphthyl. Of the aralkylgroups, C₇-C₁₀ aralkyl groups are preferred, examples of which includebenzyl, phenethyl, phenylpropyl and naphthylmethyl. Also included aresubstituted forms of the foregoing monovalent hydrocarbon groups inwhich one or more hydrogen atoms are replaced by halogen atoms, cyanogroups or the like. In average compositional formula (1), R¹ is mostpreferably methyl or phenyl.

The C₁-C₄ organic groups represented by R² include alkyl and alkenylgroups. OR² stands for a terminal group on a siloxane resin other thansilanol (Si—OH) groups, for example, methoxy, ethoxy, propoxy,isopropoxy and butoxy groups. Of these, methoxy and isopropoxy arepreferred because the starting reactants are readily available.

In average compositional formula (1), x, y and z are numbers satisfyingthe range: 0.8≦x≦1.5, 0≦y≦0.3, 0.001≦z≦0.5, and 0.801≦x+y+z<2, andpreferably 0.9≦x≦1.3, 0.001≦y≦0.2, 0.01≦z≦0.3, and 0.911≦x+y+z≦1.8. If“x” indicative of the content of R¹ is less than 0.8, the polysiloxanebecomes harder and less crack resistant. If “x” is more than 1.5, ahigher content of organic groups enhances hydrophobicity andflexibility, losing the anti-cracking effect and leading to defectiveappearance due to cissing. If “y” indicative of the content of OR² ismore than 0.3, the polysiloxane tends to have a more content of terminalgroups and a lower molecular weight, failing to exert the anti-crackingeffect. If “z” indicative of the content of OH is more than 0.5, a moreproportion of groups participate in condensation reaction upon heatcuring, leading to a higher hardness and less crack resistance. If “z”is less than 0.001, the polysiloxane tends to have a higher meltingpoint and becomes awkward to work. The value of “z” is preferablycontrolled by tailoring the percent complete condensation of alkoxygroups to the range of 86 to 96%. There is a tendency that the meltingpoint becomes lower at a condensation of less than 86% and excessivelyhigh at a condensation of more than 96%.

The organopolysiloxane having average compositional formula (1) ascomponent (A) may also be represented by a combination of Q units(SiO_(4/2)) derived from tetrafunctional silane, T units (R¹SiO_(3/2))derived from trifunctional silane, D units (R¹SiO_(2/2)) derived fromdifunctional silane, and M units (R¹SiO_(1/2)) derived frommonofunctional silane. When the organopolysiloxane is expressed by thisnotation, desirably a molar proportion of T units (R¹SiO_(3/2)) is atleast 70 mol %, more desirably at least 75 mol %, and even moredesirably at least 80 mol %, based on the total moles of entire siloxaneunits. If the proportion of T units is less than 70 mol %, an overallprofile of hardness, adhesion and appearance may be compromised. It isnoted that the balance may consist of M, D and Q units, the sum of theseunits being desirably equal to or less than 30 mol %. With respect tothe melting point, there is a tendency that the melting point rises withan increasing proportion of Q and T units, and the melting point lowerswith an increasing proportion of D and M units. It is preferred that themolar proportion of T units (R¹SiO_(3/2)) be at least 70 mol % and Dunits account for the remaining proportion of up to 30 mol %.

The organopolysiloxane as component (A) has a melting point of 40 to130° C., and preferably 70 to 80° C. If the melting point is below 40°C., the organopolysiloxane is not solid or remains still solid with asticky surface and is difficult to transfer mold. If the melting pointis above 130° C., the organopolysiloxane loses flow and is difficult totransfer mold.

The organopolysiloxane as component (A) may be prepared throughhydrolytic condensation of an organosilane having the general formula(3):R¹ _(n)SiX_(4-n)  (3)wherein R¹ is each independently an organic group of 1 to 20 carbonatoms, preferably C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₆-C₂₀ aryl or C₇-C₂₀aralkyl group; X is a halogen atom such as chlorine or an alkoxy group,typically C₁-C₄ alkoxy group; and n is 1, 2 or 3.

In average compositional formula (3), X is preferably a halogen atom,especially chlorine because organopolysiloxanes in solid form can beprepared.

In formula (3), n is an integer of 1 to 3. Where n is 2 or 3, i.e., aplurality of R¹ are included, each R¹ may be the same or different.Herein, n=1 is preferred because organopolysiloxanes in solid form canbe prepared.

Examples of the silane compound having formula (3) includeorganotrichlorosilanes and organotrialkoxysilanes such asmethyltrichlorosilane, ethyltrichlorosilane, phenyltrichlorosilane,methylvinyldichlorosilane, vinyltrichlorosilane, diphenyldichlorosilane,methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane,ethyltriethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, andphenyltriethoxysilane; and diorganodialkoxysilanes such asdimethyldimethoxysilane, dimethyldiethoxysilane,diphenyldimethoxysilane, diphenyldiethoxysilane,methylvinyldimethoxysilane, methylphenyldimethoxysilane, andmethylphenyldiethoxysilane. Of these, methyltrichlorosilane is mostpreferred. It is also effective to use methyltrichlorosilane incombination with phenyltrichlorosilane.

For these silane compounds, a choice of a trichlorosilane ortrialkoxysilane and its amount used are preferably determined so as toproduce a silanol-bearing organopolysiloxane containing at least 70 mol% of T units.

Any standard technique may be applied when the above-mentioned silanecompound having a hydrolyzable group is hydrolyzed and condensed.Preferably the reaction is carried out in the presence of an acidcatalyst such as acetic acid, hydrochloric acid or sulfuric acid or abasic catalyst such as sodium hydroxide, potassium hydroxide ortetramethylammonium hydroxide. In an example where a silane havingchloro as the hydrolyzable group is used, hydrochloric acid forms as aresult of addition of water and serves as the catalyst whereby ahydrolytic condensate having the desired molecular weight may beobtained.

Upon hydrolysis and condensation, the amount of water added is usually0.9 to 1.6 moles and preferably 1.0 to 1.3 moles per mole of totalhydrolyzable groups (e.g., chloro groups) in the silane compound(s).Insofar as the amount of water is in the range of 0.9 to 1.6 moles, theresulting composition is effective to work and cures into a toughproduct.

Preferably the silane compound having a hydrolyzable group is hydrolyzedin an organic solvent typically selected from alcohols, ketones, esters,cellosolves, and aromatic compounds. Suitable organic solvents includealcohols such as methanol, ethanol, isopropyl alcohol, isobutyl alcohol,n-butanol, and 2-butanol and aromatic compounds such as toluene andxylene. More preferably a mixture of isopropyl alcohol and toluene isused because the resulting composition becomes effectively curable andthe cured product becomes tougher.

The reaction temperature for hydrolysis and condensation is preferably10 to 120° C., and more preferably 20 to 100° C. This reactiontemperature range ensures to produce a solid hydrolytic condensatewithout gelation so that it is ready for use in the subsequent step.

One exemplary synthesis process starting with methyltrichlorosilane isdescribed. To a solution of methyltrichlorosilane in toluene, water andisopropyl alcohol are added, whereupon partial hydrolysis takes place ata temperature between −5° C. and 100° C. Then water is added in anamount sufficient to hydrolyze the entire amount of remaining chlorogroups whereupon reaction takes place, yielding a solid silicone polymerhaving a melting point of 76° C.

The desired organopolysiloxane is obtained in this way. Theorganopolysiloxane typically has a melting point of 50 to 100° C., andpreferably 70 to 80° C. If the melting point is below 50° C. or above100° C., a problem of inefficient kneading arises in the subsequentmixing step.

B. White Pigment

Component (B) is a white pigment which is blended as a white colorantfor enhancing whiteness. In the present invention, the preferred whitepigment is titanium dioxide whose unit cell may be of the rutile,anatase or brookite type, with the rutile type being preferred. Also,the average particle size and shape of titanium dioxide are not limitedalthough an average particle size of 0.05 to 5.0 μm is generally used.Titanium dioxide may be previously surface treated with hydrated oxidesof aluminum, silicon and the like in order to enhance its compatibilitywith and dispersibility in resins and inorganic fillers.

Besides titanium dioxide, other white pigments or colorants such aspotassium titanate, zirconium oxide, zinc sulfide, zinc oxide, alumina,and magnesium oxide having an average particle size of 0.1 to 3.0 μm maybe used alone or in combination with titanium dioxide.

The amount of the white pigment blended is 3 to 200 parts, desirably 5to 150 parts, and more desirably 10 to 120 parts by weight per 100 partsby weight of component (A). Less than 3 phr of the white pigment mayfail to achieve the desired whiteness, and the resulting composition maycure into a product not having an initial reflectance value in the400-800 nm wavelength region of at least 70%, especially at least 80%and an aged reflectance value of at least 70%, especially at least 80%after a heat age test of heating at 180° C. for 24 hours. More than 200phr of the white pigment may give rise to a problem of reducing theproportion of other components which are added for the purpose ofenhancing mechanical strength. The white pigment is desirably present inan amount of 1 to 50% by weight, more desirably 5 to 30% by weight, andeven more desirably 8 to 30% by weight, based on the total weight of thesilicone resin composition.

C. Inorganic Filler

Component (C) is an inorganic filler. The filler is selected from thosecommonly used in silicone resin compositions, for example, silicas suchas fused silica, fused spherical silica, and crystalline silica,alumina, zinc oxide, silicon nitride, aluminum nitride, and boronnitride, with the proviso that the aforementioned white pigment (orwhite colorant) is excluded. These inorganic fillers may be used aloneor in admixture of two or more. The particle size and shape of inorganicfiller are not particularly limited although an average particle size of4 to 40 μm is preferred, with a particle size of 7 to 35 μm being morepreferred.

Specifically, alumina is preferably used for high thermal conductivityand high fluidity. An average particle size of 4 to 40 μm, especially 7to 35 μm is preferred for moldability and fluidity. A higher fluidity isdesirably achievable when an alumina powder of a particle sizedistribution having three peaks in a fine range of 0.4 to 1.0 μm, amedium range of 8 to 18 μm, and a coarse range of 30 to 50 μm is used.In this powder, an alumina faction falling in the fine or submicronrange of 0.4 to 1.0 μm serves as the white pigment.

As discussed above, alumina serves as the inorganic filler when it hasan average particle size of 4 to 40 μm and as the white pigment when ithas an average particle size of 0.1 to 3.0 μm. Alumina serving as thewhite pigment is effective in light reflection and specifically,effective to provide a reflectance of at least 80% in a wavelengthregion equal to or less than 400 nm. However, since compounding aluminaof the submicron range provides the composition with a noticeableviscosity buildup, it is precluded to incorporate a large amount of suchalumina.

The inorganic filler which has been surface treated with coupling agentssuch as silane and titanate coupling agents for enhancing the bondstrength between the resin and the filler may also be blended. Suitablecoupling agents include epoxy-functional alkoxysilanes such asγ-glycidoxypropyltrimethoxysilane,γ-glycidoxypropylmethyldiethoxysilane, andβ-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino-functionalalkoxysilanes such as N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane,γ-aminopropyltriethoxysilane and N-phenyl-γ-aminopropyltrimethoxysilane;and mercapto-functional alkoxysilanes such asγ-mercaptopropyltrimethoxysilane. The amount of the coupling agent usedand the surface treatment method are not particularly limited. Preferredare those treated fillers which undergo no yellowing upon exposure to150° C. or higher temperatures.

The amount of the inorganic filler blended is preferably at least 50parts, more preferably at least 100 parts, even more preferably at least150 parts by weight and up to 1,000 parts, more preferably up to 950parts, even more preferably up to 900 parts, most preferably up to 800parts by weight per 100 parts by weight of component (A). The inorganicfiller is desirably present in an amount of 40 to 92% by weight, andmore desirably 60 to 90% by weight, based on the total weight of thesilicone resin composition.

The amount of white pigment and inorganic filler combined is desirably50 to 95% by weight, and more desirably 75 to 91% by weight, based onthe total weight of the silicone resin composition. If the combinedamount is less than 50 wt %, the composition may have a highercoefficient of expansion and lower mechanical strength. More than 95 wt% may bring about a viscosity buildup and a loss of flexibility, leadingto short-shots and defectives in packages such as delamination.

D. Condensation Catalyst

Component (D) is a condensation or curing catalyst. It is a condensationcatalyst for promoting the curing of a heat curable silicone resin ascomponent (A). A particular condensation catalyst is selected whiletaking into account the stability of component (A), the hardness of acoating, non-yellowing, and curing ability. Suitable catalysts includeorganometallic catalysts such as zinc salts of organic acids, Lewis acidcatalysts, organoaluminum compounds, and organotitanium compounds. Alsoincluded are basic compounds such as trimethylbenzylammonium hydroxide,tetramethylammonium hydroxide, n-hexylamine, tributylamine,diazabicycloundecene (DBU), and dicyandiamide; metal-containingcompounds such as tetraisopropyl titanate, tetrabutyl titanate, titaniumacetylacetonate, aluminum triisobutoxide, aluminum triisopropoxide,zirconium tetra(acetylacetonate), zirconium tetrabutyrate, cobaltoctylate, cobalt acetylacetonate, iron acetylacetonate, tinacetylacetonate, dibutyltin octylate, dibutyltin laurate, zinc octylate,zinc benzoate, zinc p-tert-butylbenzoate, zinc laurate, zinc stearate,aluminum phosphate, and aluminum triisopropoxide; and aluminumtrisacetylacetonate, aluminum bisethylacetoacetate monoacetylacetonate,and organotitanium chelates such asdiisopropoxybis(ethylacetoacetato)titanium anddiisopropoxybis(ethylacetoacetato)titanium. Of these, preference isgiven to zinc octylate, zinc benzoate, zinc p-tert-butylbenzoate, zinclaurate, zinc stearate, aluminum phosphate, aluminum triisopropoxide,and organotitanium chelates. Inter alia, zinc benzoate is mostpreferred.

The condensation catalyst may be used in an amount of at least 0.0001part, specifically at least 0.001 part by weight per 100 parts by weightof component (A). Preferably the curing catalyst is used in an amount of0.01 to 10 parts, more preferably 0.1 to 6 parts by weight per 100 partsby weight of component (A). An amount of the catalyst within this rangeprovides an effective stable cure process.

In addition to the above components, the following additional componentsmay be incorporated.

E. Parting Agent

In the silicone resin composition, (E) an internal parting agent may becompounded. The internal parting agent is used for the purpose offacilitating removal of a molded part from the mold. The parting agentis added in an amount of 0.2 to 5.0% by weight of the overallcomposition. Examples of the internal parting agent include naturalwaxes such as carnauba wax, and synthetic waxes such as acid wax,polyethylene wax, and fatty acid esters. Inter alia, calcium stearatehaving a melting point of 120 to 140° C. is desired. The parting agentis effective in restraining yellowing upon exposure to high temperaturesor light and maintains good parting properties over a long period.

F. Silane Coupling Agent

In the silicone resin composition, (F) a silane coupling agent may becompounded for enhancing the bond strength between the resin and theinorganic filler. Suitable coupling agents include epoxy-functionalalkoxysilanes such as γ-glycidoxypropyltrimethoxysilane,γ-glycidoxypropylmethyldiethoxysilane, andβ-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino-functionalalkoxysilanes such as N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane,γ-aminopropyltriethoxysilane and N-phenyl-γ-aminopropyltrimethoxysilane;and mercapto-functional alkoxysilanes such asγ-mercaptopropyltrimethoxysilane. The amount of the coupling agent usedand the surface treatment method are not particularly limited.

The silane coupling agent may be used in an amount of 0.1 to 8.0 parts,preferably 0.5 to 6.0 parts by weight per 100 parts by weight ofcomponent (A). Less than 0.1 phr of the coupling agent may achieveinsufficient adhesion to substrates. More than 8.0 phr of the couplingagent may invite an extreme drop of viscosity, causing voids andflashes.

G. Adhesive Aid

In the silicone resin composition, (G) an adhesive aid may becompounded. The adhesive aid is used for enhancing adhesion and may beadded in an amount of 0.2 to 5.0% by weight based on the overallcomposition. It is a 1,3,5-triazine nucleus derivative epoxy resinhaving the compositional formula (2).

Herein, each of R⁰¹, R⁰² and R⁰³ is an organic group of 1 to 10 carbonatoms, typically alkyl group, and at least one of R⁰¹, R⁰² and R⁰³contains an epoxy group. Suitable epoxy-containing groups includeglycidyl and glycidoxy. Useful examples includetris(2,3-epoxypropyl)isocyanate wherein R⁰¹, R⁰² and R⁰³ are allglycidyl, monoallyldiglycidylisocyanurate,1-allyl-3,5-(2-methylepoxypropyl)isocuanurate,1-(2-methylpropenyl)-3,5-diglycidylisocyanurate,1-(2-methylpropenyl)-3,5-(2-methylepoxypropyl)isocyanurate. Thoseadhesive aids which do not yellow upon exposure to high temperatures orlight are desired.

Other Additives

If necessary, the silicone resin composition may further include variousother additives. For example, various whiskers, silicone powder,thermoplastic resins, thermoplastic elastomers, organic syntheticrubbers, and other additives may be used for the purposes of improvingcertain properties of the resin insofar as they do not compromise theobjects of the invention.

In the silicone resin composition, phenol, phosphorus or sulfur-basedantioxidants may be blended if desired. Even absent such antioxidants,the composition of the invention experiences less discoloration thanconventional heat-curable silicone resin compositions.

The silicone resin composition of the invention is heat curable. Forexample, it cures by heating at a temperature of 150 to 185° C. for 30to 180 seconds. This may be followed by post-curing at 150 to 180° C.for 2 to 20 hours.

The cured product obtained by curing the white heat-curable siliconeresin composition comprising essential components (A) to (D) accordingto the invention has a thermal conductivity of 1 to 10 W/mK, preferably1.5 to 7 W/mK, and more preferably 2 to 5 W/mK. A cured product with toolow a thermal conductivity fails to carry away the heat release from theoptoelectronic part, resulting in a premature degradation thereof. Anappropriate thermal conductivity is achievable by addition of the heatconductive filler described above. It is noted that a thermalconductivity is measured by the method to be described later.

Specifically, an appropriate thermal conductivity is achieved byselecting a highly heat conductive one(s) as white pigment (B) and/orinorganic filler (C), especially as inorganic filler (C) andincorporating a relatively large amount thereof. Preferred highly heatconductive fillers include alumina, zinc oxide, silicon nitride,aluminum nitride, and boron nitride, with alumina being most preferred.

The amount of white pigment and inorganic filler combined is desirably50 to 95% by weight, and more desirably 75 to 91% by weight, based onthe total weight of the silicone resin composition. If the combinedamount of white pigment and inorganic filler is less than 50 wt %, thecomposition may not be fully heat conductive. Increasing the fillerloading beyond 95 wt % may render the composition less flowable anddifficult to mold.

In the preferred embodiment, white pigment (B) and inorganic filler (C)are combined to form a powder having a particle size distribution havingmaximum peaks in the three ranges of 0.4 to 1.0 μm, specifically 0.4 to0.7 μm, 8 to 18 μm, specifically 12 to 18 μm, and 30 to 50 μm,specifically 30 to 38 μm.

Alumina is advantageously used, in particular, from the standpoint ofeffective heat transfer. When alumina is used as white pigment (B)and/or inorganic filler (C), an alumina powder is preferred whichcontains 3 to 20 wt %, specifically 7 to 18 wt % of a fine aluminafraction in the fine range of average particle size 0.4 to 1.0 μm, 30 to70 wt %, specifically 40 to 65 wt % of a medium alumina fraction in themedium range of average particle size 8 to 18 μm, and 10 to 40 wt %,specifically 20 to 30 wt % of a coarse alumina fraction in the coarserange of average particle size 30 to 50 μm. Outside the range,sufficient flow may not be obtained.

In a preferred embodiment, the cured product obtained by curing thewhite heat-curable silicone resin composition has a light reflectance atwavelength 450 nm, wherein an initial (just as molded) value of lightreflectance is at least 70%, preferably at least 80%, and morepreferably at least 85%, and an aged value of light reflectance after aheat age test of heating at 180° C. for 24 hours is at least 70%,preferably at least 80%, and more preferably at least 85%. The curedproduct with a reflectance value of less than 70%, when used as anoptoelectronic part case, suffers from a shortened service lifetime.Also preferably, after exposure to a high-pressure mercury lamp of 365nm peak wavelength (60 mW/cm) for 24 hours, the cured product has areflectance value of at least 70%, preferably at least 80%, and morepreferably at least 85% at wavelength 450 nm.

It is understood that a reflectance value in the range may be achievedby using a silanol-bearing organopolysiloxane of formula (1) ascomponent (A) and incorporating a specific amount of the white pigment,typically titanium oxide.

An optoelectronic part case may be formed by molding the silicone resincomposition of the invention. The case encloses and holds in itsinterior an optoelectronic part, such as LED encapsulated with atransparent resin such as a silicone resin or epoxy resin. The interfacebetween the case and the transparent resin with which LED isencapsulated becomes a reflecting surface (reflector).

Accordingly, the silicone resin composition of the invention can beeffectively utilized as a case for optoelectronic devices, typicallyLEDs, and an encapsulant or sealant for photocouplers.

FIG. 1 illustrates in cross section an LED reflector as one exemplaryoptoelectronic device using a case of the silicone resin composition ofthe invention. In the LED package shown in FIG. 1, a semiconductor part1 composed of compound semiconductor is die-bonded to a lead frame 2 andwire-bonded to another lead frame (not shown) via a bonding wire 3. Alight receiving semiconductor part (not shown) which is die-bonded to alead frame (not shown) and wire-bonded to another lead frame (not shown)via a bonding wire (not shown) is opposed to the semiconductor part 1. Atransparent encapsulant resin 4 fills in between the semiconductorparts. The semiconductor part encapsulated with the encapsulant resin 4is further encapsulated or enclosed in a case 5 made of the siliconeresin composition of the invention in the cured state. The case servesas a white reflector. A lens 6 is disposed on top of the encapsulantresin 4.

The method of molding or encapsulating the silicone resin compositionaround the semiconductor part is most often transfer molding orcompression molding. Specifically, transfer molding is carried out byfeeding the silicone resin composition to a transfer molding machine andmolding under a pressure of 5 to 20 N/mm², preferably at a temperatureof 120 to 190° C. for 30 to 500 seconds, and more preferably at 150 to185° C. for 30 to 180 seconds. Compression molding is carried out byfeeding the silicone resin composition to a compression molding machineand molding preferably at a temperature of 120 to 190° C. for 30 to 600seconds, and more preferably at 130 to 160° C. for 120 to 300 seconds.Either of these molding methods may be followed by post-curing at 150 to185° C. for 2 to 20 hours.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation. All parts are by weight (pbw).

Examples 1 to 6 & Comparative Examples 1 to 4

The raw materials used in Examples and Comparative Examples are firstdescribed.

A. Heat-Curable Organopolysiloxane

Synthesis Example

A 1-L flask was charged with 100 parts of methyltrichlorosilane and 200parts of toluene. Under ice cooling, a mixture of 8 parts of water and60 parts of isopropyl alcohol was added dropwise over 5 to 20 hourswhile maintaining an internal temperature between −5° C. and 0° C. Thereaction mixture was then heated and stirred at the reflux temperaturefor 20 minutes. The reaction mixture was cooled to room temperature,after which 12 parts of water was added dropwise below 30° C. over 30minutes, followed by 20 minutes of stirring. Further 25 parts of waterwas added dropwise, after which the mixture was stirred at 40-45° C. for60 minutes. Thereafter, 200 parts of water was added whereupon theorganic layer was separated. The organic layer was washed until neutral.This was followed by azeotropic dehydration, filtration and vacuumstripping, yielding 36.0 parts of a heat-curable organopolysiloxane(A-1) having the following formula as a colorless transparent solid(m.p. 76° C.).(CH₃)_(1.0)Si(OC₃H₇)_(0.07)(OH)_(0.10)O_(1.4)B. White Pigment and C. Inorganic Filler

-   B-1: titanium dioxide, rutile type, average particle size 0.28 μm    (CR-95 by Ishihara Sangyo K. K.)-   C-1: alumina A, a mixture with an average particle size of 10 μm,    consisting of 15 wt % of alumina with an average particle size of    0.5 μm (AO-502 by Admatechs Co., Ltd.), 60 wt % of alumina with an    average particle size of 10 μm (AO-41R by Admatechs Co., Ltd.), and    25 wt % of alumina with an average particle size of 28 μm (CB-A30S    by Showa Denko K.K.)-   C-2: alumina B, a mixture with an average particle size of 11 μm,    consisting of 10 wt % of alumina with an average particle size of    0.5 μm (AO-502 by Admatechs Co., Ltd.), 65 wt % of alumina with an    average particle size of 10 μm (AO-41R by Admatechs Co., Ltd.), and    25 wt % of alumina with an average particle size of 38 μm (CB-A40 by    Showa Denko K.K.)-   C-3: alumina C, average particle size 50 μm (CB-A50S by Showa Denko    K.K.)-   C-4: spherical fused silica, average particle size 30 μm (FB-570 by    Denki Kagaku Kogyo K. K.)    D. Condensation Catalyst    D-1: zinc benzoate (Wako Pure Chemical Industries, Ltd.)

White silicone resin compositions were prepared by blending (A)heat-curable organopolysiloxane, (B) white pigment, (C) inorganicfiller, and (D) condensation catalyst in accordance with the formulationshown in Tables 1 and 2, milling the mixture on a roll mill, cooling andgrinding.

These compositions were examined for various properties by the followingtests, with the results shown in Tables 1 and 2. In all runs, thecompositions were molded on a transfer molding machine.

Spiral Flow

A spiral flow was measured by molding the composition at 175° C. and6.86 MPa for 90 seconds in a standard mold.

Hardness as Molded

The composition was molded at 175° C. and 6.86 MPa for 90 seconds into abar of 10 mm×4 mm×100 mm. The hardness of the bar as hot molded wasmeasured by a Shore D durometer.

Flexural Strength and Flexural Modulus

A specimen was molded in a mold in accordance with JIS K6911 at 175° C.and 6.9 N/mm² for 90 seconds before it was measured for flexuralstrength and flexural modulus at room temperature (25° C.).

Thermal Yellowing Resistance

A disc of 50 mm diameter and 3 mm thickness was molded at 175° C. and6.9 N/mm² for 90 seconds. The disc was held at 180° C. for 24 hours orsubjected to IR reflow. Any change of the disc surface was visuallyinspected as a measure of thermal yellowing resistance.

Light Reflectance

A disc of 50 mm diameter and 3 mm thickness was molded at 175° C. and6.9 N/mm² for 90 seconds. The disc was held at 180° C. for 24 hours orirradiated with UV radiation for 24 hours under a high-pressure mercurylamp (60 mW/cm) with 365 nm peak wavelength. Immediately after molding,after 24 hours of hot holding, and after 24 hours of UV exposure, thedisc (cured product) was measured for reflectance at wavelength 450 nm,using a spectrophotometer X-Rite 8200 (distributed by SDG Co., Ltd.).

Thermal Conductivity

A specimen of 50 mm diameter and 6 mm thickness was prepared by transfermolding the composition at 175° C. and 6.9 N/mm² for 90 seconds andpost-curing at 180° C. for 4 hours. The specimen was sandwiched betweentop and bottom heaters together with a calorimeter and closely contactedtherewith under pneumatic pressure. A thermal conductance wasautomatically computed from a temperature difference between oppositesurfaces of the specimen and an output of the calorimeter at the timewhen the steady state at 50° C. was reached. The value of thermalconductance multiplied by the thickness of the specimen gives a thermalconductivity.

TABLE 1 Example Composition (pbw) 1 2 3 4 5 6 A. Heat-curableorganopolysiloxane A-1 100 100 100 100 100 100 B. & C. White pigmentTitanium dioxide B-1 100 100 100 100 100 100 and inorganic Alumina A C-1300 250 200 200 filler Alumina B C-2 300 400 Alumina C C-3 Spherical C-4100 fused silica D. Condensation Zinc benzoate D-1 3 3 3 3 3 3 catalystTest results Spiral flow (cm) 35 40 60 55 35 25 Flexural strength @RT(N/mm²) 70 65 61 65 70 68 Flexural modulus @RT (N/mm²) 10000 9100 82009400 10000 15000 Hardness as molded 70 71 65 65 70 75 Thermal Initial(Appearance) white white white white white white yellowing 180° C./24 hrholding white white white white white white (Appearance) IR reflow(Appearance) white white white white white white Reflectance Initial (%)93 93 93 93 93 93 180° C./24 hr holding (%) 92 93 93 93 93 93 24 hr UVexposure (%) 93 93 93 93 93 93 Thermal conductivity (W/mK) 4.5 3.1 2.32.6 4.5 4.9

TABLE 2 Comparative Example Composition (pbw) 1 2 3 4 A. Heat-curableorganopolysiloxane A-1 100 100 100 100 B. & C. White pigment Titaniumdioxide B-1 100 100 100 100 and inorganic Alumina A C-1 filler Alumina BC-2 Alumina C C-3 300 Spherical C-4 300 200 fused silica D. CondensationZinc benzoate D-1 3 3 3 3 catalyst Test results Spiral flow (cm) 40 6010 70 Flexural strength @RT (N/mm²) 64 60 samples 43 Flexural modulus@RT (N/mm²) 9500 8000 could 4000 Hardness as molded 68 61 not be 60Thermal Initial (Appearance) white white prepared white yellowing 180°C./24 hr holding white white white (Appearance) IR reflow (Appearance)white white white Reflectance Initial (%) 90 91 93 180° C./24 hr holding(%) 90 90 93 24 hr UV exposure (%) 90 90 93 Thermal conductivity (W/mK)0.9 0.8 0.4

The test results of Examples 1 to 6 in Table 1 demonstrate that thewhite heat-curable silicone resin compositions in the first embodimentare effectively flowable and curable and cure into products havingmechanical strength, white color, heat resistance, light resistance, anda high thermal conductivity. It is proven that an optoelectronic packagein which an LED is enclosed in the cured product (reflector) of thecomposition performs well.

Examples 7 to 9 & Comparative Examples 5 to 6

The raw materials used in Examples and Comparative Examples aredescribed below.

A. Heat-Curable Organopolysiloxane

This is the same as in Examples 1 to 6.

B. White Pigment

B-1: titanium dioxide (CR-95 by Ishihara Sangyo K. K.)

C. Inorganic Filler

C-1: alumina A (the same as in Examples 1 to 6)

D. Condensation Catalyst

D-1: zinc benzoate (Wako Pure Chemical Industries, Ltd.)

E. Parting Agent

E-1: calcium stearate (Wako Pure Chemical Industries, Ltd.)

F. Silane Coupling Agent

-   F-1: 3-mercaptopropyltriethoxysilane (KBM-803 by Shin-Etsu Chemical    Co., Ltd.)-   F-2: 3-glycidoxypropyltriethoxysilane (KBM-403 by Shin-Etsu Chemical    Co., Ltd.)    G. Adhesive Aid-   G-1: tris(2,3-epoxypropyl)isocyanurate (TEPIC-S by Nissan Chemical    Industries, Ltd., epoxy equivalent 100)

White silicone resin compositions were prepared by blending theforegoing components in accordance with the formulation shown in Table3. They were examined by the following adhesion test, with the resultsshown in Table 3.

Adhesion Test

The silicone resin composition was molded on 20×20 mm frame substratesof three metals (Ag, Cu, Pd) at 175° C. and 70 kgf/mm² for 90 secondsand post cured at 180° C. for 4 hours, forming test pieces. They weresubjected three times to IR reflow at a temperature of 260° C. Using amulti-purpose bond tester DAGE Series 4000, the bond strength at roomtemperature was measured while pulling the test piece at a speed of 0.2mm/sec.

TABLE 3 Comparative Example Example Composition (pbw) 7 8 9 5 6 A.Heat-curable organopolysiloxane A-1 100 100 100 100 100 B. WhiteTitanium dioxide B-1 100 100 100 pigment C. Inorganic Alumina A C-1 200200 200 filler Spherical fused C-4 200 200 silica D. Condensation Zincbenzoate D-1 1.4 1.4 1.4 1.4 1.4 catalyst E. Parting Calcium stearateE-1 2.4 2.4 2.4 2.4 2.4 agent F. Silane KBM-803 F-1 0.5 0.5 0.5 0 0.5coupling KBM-403 F-2 2 2 2 0 2 agent G. Adhesive aid TEPIC-S G-1 2.5 510 0 0 Test results Ag bond After post-cure (MPa) 5.1 7.2 6.1 1.5 3.0test After IR reflow (MPa) 3.5 6.7 5.7 0.9 1.8 Cu bond After post-cure(MPa) 3.9 5.9 6.2 1.8 3.9 test After IR reflow (MPa) 5.4 5.7 6.2 1.2 3.0Pd bond After post-cure (MPa) 3.6 5.0 5.3 1.2 3.9 test After IR reflow(MPa) 3.8 4.5 5.6 0.7 3.2 Thermal conductivity (W/mK) 2.5 2.5 2.5 0.80.8

The test results in Table 3 demonstrate that the white heat-curablesilicone resin compositions cure to metal substrates at a high bondstrength with an excellent heat conductivity. It is proven that anoptoelectronic package in which an LED is enclosed in the cured product(reflector) of the composition performs well.

Japanese Patent Application Nos. 2008-150328 and 2008-150356 areincorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A white heat-curable silicone resin for forming optoelectronic partcases, comprising (A) a heat-curable organopolysiloxane having theaverage compositional formula (1):R¹ _(x)Si(OR²)_(y)(OH)_(z)O₄-_(x)-_(y)-_(z)y_(/2) (1) wherein R¹ is eachindependently an organic group of 1to 20carbon atoms, R² is eachindependently an organic group of 1to 4carbon atoms, x, y and z arenumbers satisfying 0.8 <x <1.5, 0<y <0.3, 0.001 <z <0.5and 0.801 <x+y+z<2, (B) a white pigment, (C) an inorganic filler excluding the whitepigment, and (D) a condensation catalyst, wherein a particulate mixtureof the white pigment (B) and the inorganic filler (C) has a particlesize distribution having maximum peaks in the three ranges of 0.4 to 1.0μm, 8 to 18 μm, and 30 to 50 μm, said composition having a thermalconductivity of 1 to 10 W/mK.
 2. A white heat-curable silicone resin forforming optoelectronic part cases, comprising (A) a heat-curableorganopolysiloxane, (B) a white pigment, (C) an inorganic fillerexcluding the white pigment, and (D) a condensation catalyst, (E), in anamount of 0.2 to 5.0% by weight based on the total weight of thecomposition, a parting agent comprising calcium stearate having amelting point of 120 to 140° C. wherein a particulate mixture of thewhite pigment (B) and the inorganic filler (C) has a particle sizedistribution having maximum peaks in the three ranges of 0.4 to 1.0 μm,8 to 18 μm, and 30 to 50 μm, said composition having a thermalconductivity of 1 to 10 W/mK.
 3. The composition of claim 1 wherein thewhite pigment (B) is one or more members selected from the groupconsisting of titanium dioxide having an average particle size of 0.05to 5.0 μm, and potassium titanate, zirconium oxide, zinc sulfide, zincoxide, alumina, and magnesium oxide, each having an average particlesize of 0.1 to 3.0 μm, and the inorganic filler (C) is one or moremembers selected from the group consisting of alumina, zinc oxide,silicon nitride, aluminum nitride and boron nitride, each having anaverage particle size of 4 to 40 μm.
 4. The composition of claim 1 orclaim 2, wherein the white pigment (B) and the inorganic filler (C) arepresent in a total amount of 50 to 95% by weight based on the totalweight of the composition.
 5. The composition of claim 1 or claim 2,wherein the white pigment (B) is titanium dioxide.
 6. The composition ofclaim 1 or claim 2 which further comprises (F) a silane coupling agentand/or (G) an adhesive aid which is a 1,3,5-triazine nucleus derivativeepoxy resin having the compositional formula (2):

wherein R⁰¹, R⁰² and R⁰³ each are an organic group of 1 to 10 carbonatoms, at least one of R⁰¹, R⁰² and R⁰³ containing an epoxy group. 7.The composition of claim 1 or claim 2, wherein the condensation catalyst(D) is an organometallic condensation catalyst.
 8. The composition ofclaim 7 wherein the organometallic condensation catalyst is zincbenzoate.
 9. An optoelectronic part case comprising a silicone resincomposition in the cured state, said composition having a thermalconductivity of 1 to 10 W/mK and comprising (A) a heat-curableorganopolysiloxane, (B) a white pigment, (C) an inorganic fillerexcluding the white pigment, and (D) a condensation catalyst, wherein aparticulate mixture of the white pigment (B) and the inorganic filler(C) has a particle size distribution having maximum peaks in the threeranges of 0.4 to 1.0 μm, 8 to 18 μm, and 30 to 50 μm, in which atransparent resin-encapsulated optoelectronic part is enclosed.
 10. Thecomposition of claim 9, wherein the white pigment (B) is one or moremembers selected from the group consisting of titanium dioxide having anaverage particle size of 0.05 to 5.0 μm, and potassium titanate,zirconium oxide, zinc sulfide, zinc oxide, alumina, and magnesium oxide,each having an average particle size of 0.1 to 3.0 μm, and the inorganicfiller (C) is one or more members selected from the group consisting ofalumina, zinc oxide, silicon nitride, aluminum nitride and boronnitride, each having an average particle size of 4 to 40 μm.
 11. Thecomposition of claim 9, wherein the white pigment (B) and the inorganicfiller (C) are present in a total amount of 50 to 95% by weight based onthe total weight of the composition.
 12. The composition of claim 9,wherein the silicone resin composition comprises (A) a heat-curableorganopolysiloxane having the average compositional formula (1):R¹ _(x)Si(OR²)_(y)(OH)_(z)O_((4-x-y-z)/2)  (1) wherein R¹ is eachindependently an organic group of 1 to 20 carbon atoms, R² is eachindependently an organic group of 1 to 4 carbon atoms, x, y and z arenumbers satisfying 0.8 ≦ x ≦ 1.5, 0 ≦y ≦ 0.3, 0.001 ≦z ≦0.5, and 0.801≦x+y+z <2, (B) a white pigment, (C) an inorganic filler excluding thewhite pigment, and (D) a condensation catalyst, wherein a particulatemixture of the white pigment (B) and the inorganic filler (C) has aparticle size distribution having maximum peaks in the three ranges of0.4 to 1.0 μm, 8 to 18 μm, and 30 to 50 μm, said composition having athermal conductivity of 1 to 10 W/mK.
 13. The composition of claim 9,wherein the white pigment (B) is titanium dioxide.
 14. The compositionof claim 9, wherein the silicone resin composition comprises (A) aheat-curable organopolysiloxane, (B) a white pigment, (C) an inorganicfiller excluding the white pigment, (D) a condensation catalyst, and(E), in an amount of 0.2 to 5.0% by weight based on the total weight ofthe composition, a parting agent comprising calcium stearate having amelting point of 120 to 140° C., wherein a particulate mixture of thewhite pigment (B) and the inorganic filler (C) has a particle sizedistribution having maximum peaks in the three ranges of 0.4 to 1.0 μm,8 to 18 μm, and 30 to 50 μm, said composition having a thermalconductivity of 1 to 10 W/mK.
 15. The composition of claim 9 whichfurther comprises (F) a silane coupling agent and/or (G) an adhesive aidwhich is a 1,3,5-triazine nucleus derivative epoxy resin having thecompositional formula (2):

wherein R⁰¹, R⁰² and R⁰³ each are an organic group of 1 to 10 carbonatoms, at least one of R⁰¹, R⁰² and R⁰³ containing an epoxy group. 16.The composition of claim 9, wherein the condensation catalyst (D) is anorganometallic condensation catalyst.